Driving Circuit for a Light-Emitting Diode Backlight and Method for Driving the Same

ABSTRACT

A driving circuit for an LED backlight with a first and second segment has a first terminal, a second terminal for sensing a first signal, a third terminal for connecting to a first current generating unit for providing a first driving current, a first current regulation circuit, a fourth terminal for sensing a second signal, a fifth terminal for connecting to a second current generating unit, a second current regulation circuit for controlling the second current generating unit, and a control unit which is prepared to adjust the supply voltage and to adjust the first and/or second driving current in dependence on the first and the second signal by means of the first and/or the second current regulation circuits.

This application is a Continuation of: PCT application numberPCT/EP2017/053116, filed Feb. 13, 2017, which claims priority toEuropean application number EP16159637.4, filed Mar. 10, 2016, both ofwhich are owned by a common assignee and are herein incorporated byreference in their entirety.

TECHNICAL FIELD

The present application pertains to the field of LED backlights whichare used in television sets for lighting the liquid crystal display,LCD.

BACKGROUND

Modern TV sets realize a high resolution. For example, ultra-highdefinition, UHD, TV sets have a resolution of 4,000 pixels. Thebacklight in such a TV set is implemented by a number of blocks orsegments and each backlight segment lights a section of the LCD which ispreferably of rectangular shape. Each segment of the backlight iscontrolled independently so that it can be dimmed in accordance with thepicture content displayed on the LCD. This is known as local dimming.The number of segments varies between 4 and 512, for example. Each ofthese segments, also designated as channels, is usually formed by onestring of LEDs, the string being controlled independently.

SUMMARY

The present application starts out from a state of the implementation ofan LED backlight which has a single power supply for powering allsegments of the LED backlight. Said power supply can be regulated in away that the segment with the highest voltage drop defines the level ofthe supply power. This implies that the other segments with a smallervoltage drop have to dissipate the surplus energy. The heat generated insaid segments requires additional external heatsinks. Consequently, theefficiency of the LED backlight is poor.

It is therefore an objective to define a driving circuit for an LEDbacklight and a method for driving the same which enable enhancedefficiency of the LED backlight.

The objective is achieved by the subject-matter of the independentclaims. Embodiments and developments are subject-matter of the dependentclaims.

The definitions as described above also apply to the followingdescription unless stated otherwise.

In one embodiment, a driving circuit for a light-emitting diode, LED,backlight with at least a first and a second segment has a first, asecond, a third, a fourth and a fifth terminal, a first and a secondcurrent regulation circuit and a control unit. The first terminal isadapted for connecting to a power supply unit which in turn can beconnected to a first connection point of the first and the secondsegment for providing a supply voltage. The second terminal is preparedfor connecting to a second connection point of the first segment and forsensing a first signal. The third terminal is prepared for connecting toa first current generating unit which is adapted for providing a firstdriving current to the first segment. The first current regulationcircuit is coupled to the second and the third terminal in order tocontrol the first current generating unit. The fourth terminal isprepared for connecting to a second connection point of the secondsegment and for sensing a second signal. The fifth terminal is adaptedfor connecting to a second current generating unit for providing asecond driving current to the second segment. The second currentregulation circuit is coupled to the fourth and the fifth terminal inorder to control the second current generating unit. The control unit isprepared to adjust the supply voltage and to adjust the first and/or thesecond driving current in dependence on the first and/or the secondsignal by means of the first and/or the second current regulationcircuits.

The supply voltage is provided to the first and the second segment bythe power supply unit. Additionally, the first segment is provided withthe first driving current by the first current generating unit undercontrol of the first current regulation circuit. The second segment isprovided with the second driving current by means of the second currentgenerating unit which is under control of the second current regulationcircuit. The control unit evaluates the first and the second signal andcorrespondingly adjusts the supply voltage. Subsequently, the controlunit adjusts the first and/or the second driving current depending onmeasurement results from the first and second signal using the firstand/or the second current regulation circuits.

By means of the adjustment of the supply voltage and subsequentcontinuous measurement of the first and second signals and correspondingadjustment of the first and/or second driving current, a difference involtage drops between the first and the second segment is reduced.Consequently, less power or heat has to be dissipated in the first andsecond segment. The driving circuit enables improving the overallefficiency of the driven LED backlight.

In a development, the first signal is sensed depending on a voltage dropacross the first segment and the second signal is sensed depending on avoltage drop across the second segment.

The voltage drops across the first and the second segment each depend onthe level of the supply voltage, the level of the driving current, aswell as the number of LEDs and their forward voltage.

In another development, the first current regulation circuit is adaptedto provide a first pulse width modulated signal to the second terminalfor adjusting a duty cycle of the first driving current and to provide afirst current control signal to the third terminal for adjusting a levelof the first driving current. The second current regulation circuit isadapted to provide a second pulse width modulated signal to the fourthterminal for adjusting a duty cycle of the second driving current and toprovide a second current control signal to the fifth terminal foradjusting a level of the second driving current.

Based on continuously measuring the voltage drops across the first andthe second segments after adjustment of the supply voltage, the controlunit realizes an adjustment of the first and/or the second drivingcurrent by instructing the first and/or the second current regulationcircuit to provide the first and/or second current control signal suchthat the level of the first and/or the second driving current is reducedand at the same time to provide the first and/or the second pulse widthmodulated signal such that the duty cycle of the first and/or the seconddriving current is increased.

Consequently, each segment is provided with a driving current which isregulated to the envisaged operating point of the segment. Thereby lesspower and heat have to be dissipated in the segments.

In one embodiment an LED backlight arrangement comprises the drivingcircuit as described above, a power supply unit, the first and thesecond segment, as well as the first and the second current generatingunits. The power supply is coupled to the first terminal of the drivingcircuit. The first segment has a first serial connection of at least twolight-emitting diodes, LEDs. The first serial connection is coupledbetween the first and the second connection point of the first segment.The second segment comprises a second serial connection of at least twoLEDs. Therein, the serial connection is coupled between the first andthe second connection point of the second segment. The first currentgenerating unit is coupled to the second connection point of the firstsegment in series to the first segment. The second current generatingunit is connected to the second connection point of the second segmentin series to the second segment.

The power supply unit supplies first and second segments in a parallelconnection. A voltage drop across the first segment which is reflectedin the first signal amounts to the sum of forward voltages of the LEDsin the first segment. The voltage drop across the second segment, whichis sensed by means of the second signal, amounts to a sum of the forwardvoltages of the LEDs which make up the second segment. Without anyeffort in binning of the LEDs used in the first and the second segmentin order to have equal forward voltages in all LEDs employed, thevoltage drop across the first segment will differ from a voltage dropacross the second segment. This difference or variance in voltage dropsacross the segments of the LED backlight arrangement is advantageouslycompensated by using the driving circuit described above. Therefore,less effort has to be spent on cooling circuit elements in the first andthe second segment which reduces the area required for implementing thedriving circuit, e.g. in a printed circuit board, PCB. Externalheatsinks are no longer necessary. Furthermore, cheaper LEDs can be usedas binning of the LEDs becomes redundant with the proposed drivingcircuit in the LED backlight arrangement.

In a development, the first current generating unit comprises a firstadjustable current sink and a first transistor switch. The secondcurrent generating unit comprises a second adjustable current sink and asecond transistor switch.

The first and/or second adjustable current sink can also be realized bya current source or a resistor, respectively.

In a further development, a controlled terminal of the first transistorswitch is connected to the second terminal of the driving circuit and acontrolled section of the first transistor switch is coupled between thesecond connection point of the first segment and the third terminal ofthe driving circuit. A control terminal of the second transistor switchis connected to the fourth terminal of the driving circuit and acontrolled section of the second transistor switch is coupled betweenthe second connection point of the second segment and the fifth terminalof the driving circuit.

In another development, the first adjustable current sink is connectedbetween the third terminal of the driving circuit and a referencepotential terminal. The second adjustable current sink is connectedbetween the fifth terminal of the driving circuit and the referencepotential terminal.

In another embodiment the first and the second transistor switch eachcomprises a metal oxide semiconductor field effect transistor, MOSFET,or a bipolar transistor.

Whenever MOSFETs are used, the first signal represents the gatepotential of the first transistor switch. The second signal representsthe gate potential of the second transistor switch. According to thesecond option in which bipolar transistors are used, the first signalreflects the base current of the first transistor switch and the secondsignal represents the base current of the second transistor switch.

In one embodiment a method for driving an LED backlight with at least afirst and a second segment comprises the following steps:

-   -   providing a supply voltage and a first driving current to the        first segment,    -   providing the supply voltage and a second driving current to the        second segment,    -   reducing a level of the supply voltage until a specifiable        second level of the supply voltage is reached or until a first        predefined condition in the first or the second segment if        fulfilled,    -   sensing a first signal in dependence on a voltage drop across        the first segment,    -   sensing a second signal in dependence on a voltage drop across        the second segment,    -   determining the maximum of the voltages dropping across the        first and the second segment by evaluating the first and the        second signal,    -   reducing the driving current of the segment with the maximum        voltage drop until a second predefined condition in the first or        the second segment is reached.

By reducing the supply voltage and subsequently regulating the drivingcurrent of the segment with the maximum voltage drop, a variance involtage drops between the first and the second segment can becompensated. Consequently, the amount of excess energy that normallyarises from such variance in voltage drop is greatly reduced whichresults in an improved overall efficiency in driving the LED backlight.

The method may be implemented in the driving circuit for an LEDbacklight specified above.

Possibly, the LED backlight which is driven by the proposed method hasmore than two segments. In that case for each additional segment asignal in dependence on a voltage drop across said segment is sensed.The maximum of all voltage drops across the segments comprised by theLED backlight is determined by evaluating all of these signals.Subsequently, the driving current of the segment with the maximumvoltage drop is reduced. It is to be noted that in an LED backlight withmore than two segments, more than one segment may have its drivingcurrent reduced due to a voltage drop the amount of which is close tothe determined maximum.

In a development while reducing the level of the supply voltage, thefirst predefined condition in the first or the second segment isfulfilled as soon as the first or the second signal is below apredefined threshold.

Said threshold is predefined, for example, as the headroom voltage ofthe segments of the LED backlight which is driven by the method. Thereinthe headroom voltage refers to the amount of additional voltage or thevoltage reserve which allows for correct operation of the LEDs in thesegments.

Consequently, the supply voltage is reduced either by a specifiableamount or until a minimum voltage level with which correct operation ofthe LEDs in the segments is still assured.

The specifiable amount can be determined beforehand taking into accountthe number of LEDs in a segment and their forward voltages, for example.

In a further development reducing the level of the supply voltage iscompleted stepwise. Said reducing comprises after each step of reducingthe level of the supply voltage sensing the first signal and sensing thesecond signal and comparing the first signal and the second signal withthe predefined threshold.

In this way it is guaranteed that the supply voltage is not reducedbelow a minimum level for correct operation.

In another development the supply voltage and the first driving currentare provided each at a predefined level. The first driving current isprovided in a pulse width modulated manner with a predefined duty cycle.The second driving current is provided at a predefined level and in apulse width modulated manner with a predefined duty cycle.

In a further development reducing the driving current of the segmentwith the maximum voltage drop comprises reducing the level of thedriving current of the segment with the maximum voltage drop andincreasing its duty cycle.

By this, any brightness difference in the segments is avoided.

In another development while reducing the driving current of the segmentwith the maximum voltage drop the second predefined condition isfulfilled as soon as the first or the second signal reaches thepredefined threshold.

During reducing the driving current, the signal of the segment underregulation is supervised for not surpassing the threshold, i.e. theheadroom voltage of the segment. This again ensures correct operation ofall LEDs in the regulated segment.

In another development reducing the driving current of the segment withthe maximum voltage drop and increasing its duty cycle is completed in astepwise manner. Said reducing comprises after each step of reducing thelevel of the driving current of the regulated segment and increasing itsduty cycle sensing the first signal, sensing the second signal andcomparing the first signal and the second signal with the predefinedthreshold.

The text below explains the proposed driving circuit, LED backlightarrangement and driving method in detail using exemplary embodimentswith reference to the drawings. Components and circuit elements that arefunctionally identical or have the identical effect bear identicalreference numbers. In so far as circuit parts or components correspondto one another in function, a description of them will not be repeatedin each of the following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment example of the proposed LED backlightarrangement with the driving circuit;

FIG. 2 shows an embodiment example of the proposed method;

FIG. 3A shows the situation in the embodiment of FIG. 1 at the beginningof the proposed method;

FIG. 3B shows the situation in the embodiment of FIG. 1 after executionof the proposed method.

DESCRIPTION

FIG. 1 shows an embodiment example of the proposed LED backlightarrangement with the proposed driving circuit. The LED backlightarrangement comprises a driving circuit DRV, a power supply unit PS, afirst segment S1, at least a second segment S2, a first currentgenerating unit T1, CS1 and a second current generating unit T2, CS2.

The driving circuit DRV has a first terminal 11 which can be connectedto the power supply unit PS, a second terminal 12, a third terminal 13,a first current regulation circuit PWM1, a fourth terminal 22, a fifthterminal 23, a second current regulation circuit PWM2 and a control unitCTL. The first current regulation circuit PWM1 is coupled to the secondand the third terminals 12, 13. The second current regulation circuitPWM2 is coupled to the fourth and to the fifth terminal 22, 23. Thecontrol unit CTL is coupled to the first and to the second currentregulation circuit PWM1, PWM2 and to the first, second and the fourthterminal 11, 12, 22. It is to be noted that the driving circuit DRV isprepared for driving the LED backlight arrangement. Whenever the drivingcircuit DRV is used for this purpose, it is connected within the LEDbacklight arrangement as described herein.

The power supply unit PS comprises, for example, a so-called switchedmode power supply which may be implemented by a DC/DC converter or anAC/DC converter. The first segment S1 comprises a serial connection ofat least two LEDs which are coupled between a first connection point Cp1and a second connection point Cp2. The first connection point Cp1 iscoupled to an output of the power supply unit PS. The second segment S2comprises a serial connection of at least two LEDs which are coupledbetween the first connection point Cp1 and a second connection point Cp3of the second segment S2.

The first current generating unit T1, CS1, comprises a first adjustablecurrent sink CS1 and a first transistor switch T1. A control terminal ofthe first transistor switch T1 is connected to the second terminal 12 ofthe driving circuit DRV. A controlled section of the first transistorswitch T1 is coupled between the second connection point Cp2 of thefirst segment S1 and the third terminal 13 of the driving circuit DRV.The first adjustable current sink CS1 is connected between the thirdterminal 13 of the driving circuit DRV and a reference potentialterminal 10. The second adjustable current sink is connected between thefifth terminal 23 of the driving circuit DRV and the reference potentialterminal 10. A control terminal of the second transistor switch T2 isconnected to the fourth terminal 22 of the driving circuit DRV. Acontrolled section of the second transistor switch T2 is coupled betweenthe second connection point Cp3 of the second segment S2 and the fifthterminal 23 of the driving circuit DRV.

First and second transistor switches T1, T2 may each be implemented by aMOSFET. In this case the term “control terminal” refers to the gateterminal and the term “controlled section” refers to the drain sourcesection of the transistor in question. In an alternative each of thetransistor switches T1, T2 can be realized by a bipolar transistor. Inthis case the base terminal represents the control terminal and thecontrolled section extends between emitter and collector terminals.

The power supply unit PS provides a supply voltage Vled to the first andthe second segment S1, S2. Under control of the first current regulationcircuit PWM1 a first driving current I1 is supplied to the first segmentS1 by means of the first current generating unit CS1, T1. In fact, thefirst current regulation circuit PWM1 provides a first current controlsignal Scs1 to the third terminal 13 for adjusting a level of the firstdriving current I1. Furthermore, the first current regulation circuitPWM1 provides a first pulse width modulated signal Sd1 to the secondterminal 12 for adjusting a duty cycle of the first driving current I1.Consequently, the first driving current I1 is generated in the firstcurrent generating unit T1, CS1 as is known to persons skilled in theart.

In analogy to the first current generating unit CS1, T1, the secondcurrent generating unit CS2, T2 generates a second driving current I2which is provided to the second segment S2. Generation of the seconddriving current I2 is controlled by the second current regulationcircuit PWM2 which provides for this purpose a second current controlsignal Scs2 to the fifth terminal for adjusting a level of the seconddriving current I2 and a second pulse width modulated signal Sd2 to thefourth terminal 22 for adjusting a duty cycle of the second drivingcurrent I2.

Driving of the first segment S1 with the supply voltage Vled and thefirst driving current I1 leads to a voltage drop Vf1 across the firstsegment S1. Similarly, driving the second segment S2 with the supplyvoltage Vled and the second driving current I2 results in a voltage dropVf2 across the second seynient S2. The voltage drop Vf1 across the firstsegment S1 is measured at the second terminal 12 and provided as a firstsignal Sg1 to the control unit CTL of the driving circuit DRV. Thevoltage drop Vf2 across the second segment S2 is measured at the fourthterminal 22 and is provided as a second signal Sg2 to the control unitCTL of the driving circuit DRV. The control unit CTL in turn provides afeedback signal Vfb via the first terminal 11 to the power supply unitPS in order to adjust the supply voltage Vled.

Furthermore, the control unit CTL provides a first control signal Sc1 tothe first current regulation circuit PWM1 in order to adjust the firstdriving current I1. Similarly, the control unit CTL provides a secondcontrol signal Sc2 to the second current regulation circuit PWM2 inorder to adjust the second driving current I2. The first control signalSc1 is provided depending on an evaluation of the first signal Sg1. Thesecond control signal Sc2 is provided depending on an evaluation of thesecond signal Sg2. The first pulse width modulated signal Sd1 and thefirst current control signal Scs1 are provided as a function of thefirst control signal Sc1. Moreover, the second pulse width modulatedsignal Sd2 and the second current control signal Scs2 are provided as afunction of the second control signal Sc2.

Functioning of the LED backlight arrangement with the driving circuitDRV is explained below in detail by means of the method depicted in FIG.2 and the circuits of FIGS. 3A and 3B.

FIG. 2 shows an embodiment example of a method for driving an LEDbacklight with at least a first and a second segment as proposed. By wayof example, the method is implemented using the LED backlightarrangement and driving circuit depicted in FIG. 1.

The method starts out with step 101 in which the supply voltage Vled isprovided to the first segment S1. Also, the first driving current I1 isprovided to the first segment S1.

In step 102 the supply voltage Vled and the second driving current I2are provided to the second segment S2.

FIG. 3A shows details regarding voltage and current levels at thebeginning of the execution of the method. It can be seen that the supplyvoltage Vled is provided with 50.5 V. First and second current sinksCS1, CS2 are adjusted to provide the first driving current I1 and thesecond driving current I2, respectively, at a level of 100 rnA. It canfurther be discerned that the duty cycle of first and second drivingcurrents I1, I2 is pre-adjusted to 50%. The resulting voltage drop Vf1across the first segment S1 amounts to 50 V. The resulting voltage dropVf2 across the second segment S2 amounts to 47 V. Therefore, the voltageof the drain terminal of the first transistor switch T1 is at 0.5 V,whereas the voltage at the drain terminal of the second transistorswitch T2 is at 3.5 V. The regulation which is described in thefollowing steps aims at compensating this difference in the voltagedrops between the first and the second segments S1, S2.

The voltages at the drain terminals of first and second transistorswitches T1, T2 are reflected in the first and the second signal Sg1,Sg2, respectively.

In step 103 the supply voltage Vled is reduced by one step. First andsecond signals Sg1, Sg2 are sensed and are each compared to a predefinedthreshold. This predefined threshold defines the operating point of thefirst and the second transistor switch T1, T2 under a condition of whichthe LEDs of the first and the second segments S1, S2 work with thedesired brightness. In an exemplary implementation, the predefinedthreshold roughly corresponds to the threshold voltage of the first andsecond transistor switch T1, T2.

In step 104 it is checked whether the first or the second signal Sg1,Sg2 is below the predefined threshold.

Alternatively, it is tested whether the supply voltage Vled has alreadybeen reduced to a level which has been specified in advance taking intoaccount the number of LEDs in a segment and their forward voltages. Aslong as neither of these tests proves positive, the supply voltage Vledis further reduced and steps 103 and 104 are repeated. In between twosteps of reducing the supply voltage Vled settlement of the power supplyunit PS to the new value is awaited.

FIG. 3B shows that the supply voltage is consequently reduced to 47.5 V.

As soon as the first or the second signal Sg1, Sg2 is below thepredefined threshold or when the supply voltage Vled has reached thespecified lower level, first and second signals are sensed again insteps 105 and 106.

In step 107 the maximum voltage drop Vf1, Vf2 across the first or thesecond segment S1, S2 is determined by comparing first and secondsignals Sg1, Sg2.

Next, in step 108, the driving current of the segment with the maximumvoltage drop is reduced step by step. In the depicted circuit of FIG. 3Bthe first segment S1 has its driving current regulated. For thispurpose, the level of the first driving current I1 is reduced stepwiseand finally is adjusted to 90 rnA by means of the first current controlsignal Scs1. Concurrently, the duty cycle of the first driving currentI1 is adjusted to 55% by means of the first pulse width modulated signalSd1. The resulting values of voltage drops Vf1, Vf2 in the segmentsamounts to 47 V, as depicted in FIG. 3B.

After each step 108 of reducing the driving current of the segment inquestion in step 109 it is checked whether the second predefinedcondition is fulfilled, i.e. whether the first or the second signal Sg1,Sg2 reaches the level of the headroom voltage. As soon as the signal ofthe segment which has its driving current regulated has reached thethreshold, for example, 3 V, the compensation is completed and themethod ends.

It can be gathered from FIG. 3B that the voltage at the drain terminalsof first and second transistor switch T1, T2 is equalized to 0.5 V aftercompletion of the method.

In an alternative embodiment of the method, the order of the steps ismodified such that after steps 101 and 102, the first and the secondsignal is sensed according to steps 105 and 106 and the maximum voltagedrop is determined according to step 107. Subsequently, the supplyvoltage is reduced until the predefined condition is fulfilled or thesecond level is reached according to steps 103 and 104. Finally, thedriving current is reduced as of steps 108 and 109.

In another alternative embodiment of the method, the order of the stepsis modified such that after steps 101 and 102, the first and the secondsignal is sensed according to steps 105 and 106 and the maximum voltagedrop is determined according to step 107. Subsequently, the drivingcurrent is reduced as of steps 108 and 109. Finally, the supply voltageis reduced until the predefined condition is fulfilled or the secondlevel is reached according to steps 103 and 104.

It is to be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments unless described as alternative. Furthermore, equivalentsand modifications not described above may also be employed withoutdeparting from the scope of the driving circuit, LED backlightarrangement and method which are defined in the accompanying claims.

REFERENCE LIST

-   10 reference potential terminal-   11, 12, 13, 22, 23 terminal of driving circuit-   DRV driving circuit-   PWM1, PWM2 current regulation circuit-   CTL control unit-   PS power supply unit-   S1, S2 segment-   CS1, CS2 current sink-   T1, T2 transistor switch-   Cp1, Cp2, Cp3 connection point-   Vled supply voltage-   Vf1, Vf2 voltage drop-   Sg1, Sg2, Sd1, Sd2 signal-   Sc1, Sc2, Scs1, Scs2 signal-   Vfb signal-   I1, I2 driving current-   101, 102, 103, 104, 105 step-   106′ 107, 108, 109 step

What is claimed is:
 1. A driving circuit for a light-emitting diode,LED, backlight with at least a first and a second segment, the drivingcircuit having a first terminal for connecting to a power supply unitwhich in turn can be connected to a first connection point of the firstand the second segment for providing a supply voltage, a second terminalfor connecting to a second connection point of the first segment and forsensing a first signal, a third terminal for connecting to a firstcurrent generating unit for providing a first driving current to thefirst segment, a first current regulation circuit which is coupled tothe second and the third terminal in order to control the first currentgenerating unit, a fourth terminal for connecting to a second connectionpoint of the second segment and for sensing a second signal, a fifthterminal for connecting to a second current generating unit forproviding a second driving current to the second segment, a secondcurrent regulation circuit which is coupled to the fourth and to thefifth terminal in order to control the second current generating unit, acontrol unit which is prepared to adjust the supply voltage and toadjust the first and/or second driving current in dependence on thefirst and the second signal by means of the first and/or the secondcurrent regulation circuits.
 2. The driving circuit according to claim 1wherein the first signal is sensed depending on a voltage drop acrossthe first segment and the second signal is sensed depending on a voltagedrop across the second segment.
 3. The driving circuit according toclaim 1, wherein the first current regulation circuit is adapted toprovide a first pulse-width modulated signal to the second terminal foradjusting a duty-cycle of the first driving current and to provide afirst current control signal to the third terminal for adjusting a levelof the first driving current, and wherein the second current regulationcircuit is adapted to provide a second pulse-width modulated signal tothe fourth terminal for adjusting a duty-cycle of the second drivingcurrent and to provide a second current control signal to the fifthterminal for adjusting a level of the second driving current.
 4. An LEDbacklight arrangement comprising A driving circuit for a light-emittingdiode, LED, backlight with at least a first and a second segment, thedriving circuit having a first terminal for connecting to a power supplyunit which in turn can be connected to a first connection point of thefirst and the second segment for providing a supply voltage, a secondterminal for connecting to a second connection point of the firstsegment and for sensing a first signal, a third terminal for connectingto a first current generating unit for providing a first driving currentto the first segment, a first current regulation circuit which iscoupled to the second and the third terminal in order to control thefirst current generating unit, a fourth terminal for connecting to asecond connection point of the second segment and for sensing a secondsignal, a fifth terminal for connecting to a second current generatingunit for providing a second driving current to the second segment, asecond current regulation circuit which is coupled to the fourth and tothe fifth terminal in order to control the second current generatingunit, a control unit which is prepared to adjust the supply voltage andto adjust the first and/or second driving current in dependence on thefirst and the second signal by means of the first and/or the secondcurrent regulation circuits the power supply unit coupled to the firstterminal of the driving circuit, the first segment comprising a firstserial connection of at least two light-emitting diodes, the firstserial connection being coupled between the first and the secondconnection point of the first segment, the second segment comprising asecond serial connection of at least two light-emitting diodes, thesecond serial connection being coupled between the first and the secondconnection point of the second segment, the first current generatingunit coupled to the second connection point of the first segment inseries to the first segment, and the second current generating unitcoupled to the second connection point of the second segment in seriesto the second segment.
 5. The LED backlight arrangement according toclaim 4, wherein the first current generating unit comprises a firstadjustable current sink and a first transistor switch, and wherein thesecond current generating unit comprises a second adjustable currentsink and a second transistor switch.
 6. The LED backlight arrangementaccording to claim 5, wherein a control terminal of the first transistorswitch is connected to the second terminal of the driving circuit and acontrolled section of the first transistor switch is coupled between thesecond connection point of the first segment and the third terminal ofthe driving circuit, and wherein a control terminal of the secondtransistor switch is connected to the fourth terminal of the drivingcircuit and a controlled section of the second transistor switch iscoupled between the second connection point of the second segment andthe fifth terminal of the driving circuit
 7. The LED backlightarrangement according to claim 5, wherein the first adjustable currentsink is connected between the third terminal of the driving circuit anda reference potential terminal, and wherein the second adjustablecurrent sink is connected between the fifth terminal of the drivingcircuit and the reference potential terminal.
 8. The LED backlightarrangement according to any of claim 5, wherein the first and thesecond transistor switch each comprises a metal-oxide semiconductorfield-effect transistor, MOSFET, or a bipolar transistor.
 9. A methodfor driving an LED backlight with at least a first and a second segment,comprising the following steps: providing a supply voltage and a firstdriving current to the first segment, providing the supply voltage and asecond driving current to the second segment, reducing a level of thesupply voltage until a specifiable second level of the supply voltage isreached or until a first predefined condition in the first or secondsegment is fulfilled, sensing a first signal in dependence on a voltagedrop across the first segment, sensing a second signal in dependence ona voltage drop across the second segment, determining the maximum of thevoltages dropping across the first and second segment by evaluating thefirst and the second signal, reducing the driving current of the segmentwith the maximum voltage drop until a second predefined condition in thefirst or second segment is reached.
 10. The method according to claim 9,wherein while reducing the level of the supply voltage the firstpredefined condition in the first or second segment is fulfilled as soonas the first or the second signal is below a predefined threshold. 11.The method according to claim 10, wherein reducing the level of thesupply voltage is completed stepwise, and wherein reducing the level ofthe supply voltage comprises after each step of reducing the supplyvoltage sensing the first signal and sensing the second signal andcomparing the first signal and the second signal with the predefinedthreshold.
 12. The method according to claim 9, wherein the supplyvoltage and the first driving current are provided each at a predefinedlevel and the first driving current is provided in a pulse-widthmodulated manner with a predefined duty-cycle, and wherein the seconddriving current is provided at a predefined level and in a pulse-widthmodulated manner with a predefined duty-cycle.
 13. The method accordingto claim 12, wherein reducing the driving current of the segment withthe maximum voltage drop comprises reducing the level of the drivingcurrent of the segment with the maximum voltage drop and increasing itsduty cycle.
 14. The method according to claim 9, wherein while reducingthe driving current of the segment with the maximum voltage drop thesecond predefined condition is fulfilled as soon as the first or thesecond signal reaches the predefined threshold.
 15. The method accordingto claim 13, wherein reducing the driving current of the segment withthe maximum voltage drop and increasing its duty cycle is completed in astepwise manner and comprises after each step of reducing the level ofthe driving current of the segment with the maximum voltage drop andincreasing its duty cycle, sensing the first signal and sensing thesecond signal and comparing the first signal and the second signal withthe predefined threshold.